upgrade to alpine 3.3 and use --no-cache option

The new option in alpine 3.3 avoids creating `/var/cache/apk/*`.

Dockerfile

@@ -3,19 +3,19 @@
 #Usage:
 #docker run -v /var/run/docker.sock:/var/run/docker.sock -v /var/lib/docker:/var/lib/docker --rm martin/docker-cleanup-volumes [--dry-run]
 #
-FROM alpine:3.2
+FROM alpine:3.3
 
 MAINTAINER Martin van Beurden <chadoe@gmail.com>
 
 ENV DOCKER_VERSION=1.4.1
 
 #Install an up to date version of docker
-RUN apk add --update-cache curl bash grep && \
+RUN apk add --no-cache curl bash grep && \
 # the docker package in alpine disables aufs and devicemapper
     curl -sSL https://get.docker.com/builds/Linux/x86_64/docker-$DOCKER_VERSION -o /usr/bin/docker && \
     chmod +x /usr/bin/docker && \
 #cleanup
-    apk del curl && rm -rf /var/cache/apk/*
+    apk del curl
 
 #Add the cleanup script
 COPY ./docker-cleanup-volumes.sh /usr/local/bin/